1. Field of the Invention
This invention relates to charge coupled device (CCD) image sensors.
2. Description of Related Art
A CCD image sensor incorporates an array of semiconductor electric charge storage and transfer channels formed on a substrate. Each channel comprises a number of elements defined by an associated electrode structure to which electric potentials may be applied to cause electric charges stored in the channel to be transferred along the channel from element to element. In use, light representing an image is focussed onto the array so as to cause photogenerated charge of a predetermined polarity to accumulate in some of the elements of the array in a pattern corresponding to the image. The accumulated charges representing the image are then read out by transferring the charges along the channels to a read-out section of the sensor.
A well known problem that arises in use of CCD image sensors is that if more charge is generated in an element by light directed into the element than can be stored in that element, excess charge overflows into adjacent elements, this phenomenon being known as blooming.
It is therefore common practice to provide in a CCD image sensor a so-called anti-blooming drain, which serves to drain off excess photogenerated charge before it can reach adjacent elements. The form of anti-blooming drain employed depends on the form of the CCD array. In a CCD image sensor of the so-called frame transfer format wherein the channels of the array are disposed in parallel spaced relationship, an anti-blooming drain structure may be provided in the gap between each adjacent pair of channels. The drain structure includes an appropriately doped semiconductor region via which excess photogenerated charge will flow out of the device in preference to flowing to adjacent elements. To achieve such preferential flow the structure also includes means for producing a potential barrier between each element where photogenerated charge collects and the drain regions, which barrier is lower than the potential barriers along each channel which define the elements in which photogenerated charge collects.
In frame transfer format CCD image sensors of the so-called buried channel type, that is to say frame transfer sensors of the kind wherein charge is stored and transferred slightly below the surface of the semiconductor substrate, it is found that the anti-blooming drain begins to lose effectiveness at high optical overloads e.g. overloads in the region of one hundred times the optical load required to fully charge an element.